The hippocampus appears to carry out spatial memory processing and navigation. As one of the inputs to the hippocampus originates in the perirhinal cortex and the spatial behaviour is affected by lesion of the perirhinal cortex, this structure may be critical for the functioning of hippocampal place cells. To investigate this hypothesis, the firing properties of hippocampal place cells were compared between control rats and rats with perirhinal cortex lesions.
Rats were randomly assigned to control and lesion groups. Animals from both groups received recording electrode implantation and the lesion group rats received bilateral perirhinal cortex lesions. In experiment 1, the control and lesioned rats moved freely in an open field. In experiment 2, the control and lesion rats ran for reward in a linear track with either horizontal or vertical grating pattern stimulation along both sidewalls. These two experiments examined the spatial firing and movement-related firing properties of the control and lesion groups; and the theta-related firing properties of the two groups. In addition, experiment 2 investigated the influence of optic flow on these properties between the two groups. In experiment 3, the control and lesion rats were passively moved in the linear track with either a horizontal or vertical grating pattern on both sidewalls. This experiment examined the spatial firing and movement-related firing properties and also investigated the influences of optic flow, motor efferent and proprioceptive information on the firing properties of the control and lesion groups� place cells.
The perirhinal cortex lesion affected the spatial firing properties of hippocampal place cells. The place field size in the lesion group was significantly reduced compared to the control group in both open field and linear track experiments. The lesion also altered the movement-related firing properties. The positive relationship between the animal�s movement speed and place cell�s firing rate was disrupted by the perirhinal cortex lesion whether the animals freely ran in the open field or in the linear track. In the open field study, the perirhinal cortex lesion altered the theta-related firing pattern, and the lesion disrupted phase precession in the linear track experiment. Phase precession is that when a rat passes through the place field, the firing of the cell advances progressively and systematically across the phase of the theta cycle from a late to an early phase of the cycle. The lesion also induced poorer theta "quality" of the EEG recorded at the hippocampal fissure. Optic flow affected the spatial firing of hippocampal place cells. The place field size was smaller in both the control and lesion group when the animals received vertical grating pattern stimulation compared to the horizontal grating condition. Change in the levels of optic flow stimulation did not, however, influence the relationship between the animal�s movement speed and place cell�s firing rate in the control group. When the animals were passively moved in a linear track, many of the place cells of both the groups stopped firing. The remaining cells from the control and lesion groups still displayed a place field. The cells in the control group lost the positive relationship between the animal�s movement speed and place cell�s firing rate.
The perirhinal cortex lesion affected the spatial, movement- related and theta-related firing properties of hippocampal place cells. Change of optic flow had a subtle effect on the movement-related firing properties of the place cells. The PrhC lesion therefore disrupted motor efferent and proprioceptive input to the HPC rather than visual sensory information. Motor efferent / proprioceptive or vibrissae information may be conveyed from related cortex to the perirhinal cortex. This information may then project from the perirhinal cortex to the hippocampus directly or indirectly via the entorhinal cortex. Future studies could investigate the relationship between whisker stimulation and hippocampal place cell firing properties and further examine the possible role of motor efferent / proprioceptive signals in the firing of these cells.
| Identifer | oai:union.ndltd.org:ADTP/266345 |
| Date | January 2007 |
| Creators | Lu, Xiaodong, n/a |
| Publisher | University of Otago. Department of Psychology |
| Source Sets | Australiasian Digital Theses Program |
| Language | English |
| Detected Language | English |
| Rights | http://policy01.otago.ac.nz/policies/FMPro?-db=policies.fm&-format=viewpolicy.html&-lay=viewpolicy&-sortfield=Title&Type=Academic&-recid=33025&-find), Copyright Xiaodong Lu |
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